The present invention relates to a recirculating burner design preferably for use in an energy conserving process furnace of the type that may employ a recuperator for preheating the combustion air to a temperature of 600.degree.-2,400.degree. F. in a steel forging furnace.
In an energy conserving process furnace employing a recuperator, and a recirculating burner as shown in U.S. Pat. No. 4,060,379, issued Nov. 29, 1977, and assigned to the same assignee as the instant invention, there is described a recirculating burner which operates satisfactorily in the combination. However, there is described herein in accordance with this invention an improved form of a recirculating burner such as might be employed in the furnace system described in this patent. The improvements in the recirculating burner are exemplified by the following and other objects which will become more apparent upon a reading of the details thereof described hereinafter.
One object of the present invention is to provide a recirculating burner that is substantially reduced in size and that, in particular, has been shortened in length. This reduction in the size of the burner has been accomplished without any degradation in the operation or efficiency of the burner. The reduction in size now simplifies servicing of the burner and permits extracting of the burner from the furnace without requiring as substantial a clear space for purposes of burner removal as hereintofore necessary.
Another object of the present invention is to provide an improved means for controlling combustion air metering into the burner. In this connection, there is described an improved nozzle arrangement preferably employing a plug valve which provides improved linear adjustment for air intake into the burner. The improvement provides for better distribution throughout the annular flows passage entering the burner and permits a more linear relationship between the physical position of the valve and the quantity of air passing the valve as well as a lower level of leakage when the valve is in the closed position. In this connection it is noted that the U.S. Pat. No. 4,060,379 provides a nozzle for directing combustion air but does not disclose a means for control associated with the nozzle.
A further object of the present invention is to provide a recirculating burner which is adapted to receive preheated combustion air and which preferably has a flame front maintained outside of the burner in the furnace cavity to increase recirculation of furnace gases and to maximize efficiency of the burner.
Still another object of the present invention is to provide an improved recirculating burner which lowers energy requirements necessary to maintain a desired temperature such as in the process furnace applications and other similar applications.
A still further purpose of the invention is to provide ample mixing within the furnace cavity to avoid isolated stratified pockets of gas which cause non-uniform heating of the thermal energy absorbing surfaces of the furnace.
Still a further object of the present invention is to provide a recirculating burner which is operable at a relatively high temperature of intake air to thus achieve considerable fuel savings and optimize the combustion process.
Another object of the present invention is to provide an improved internally recirculating burner design which operates with a low level of smoke and which is able to operate with either ambient or highly preheated air even at temperatures in the range of 600.degree. F. to 2,400.degree. F. or possibly higher. Although it is preferred that highly preheated air be used, it has been found that there is improved furnace efficiency even when combustion air at ambient temperature is employed.
A further object of this invention is to provide a burner capable of operating on or near the optimum fuel air ratio or stoichiometric ratio without generating excessive quantities of nitrogen oxide as well as particulate (smoke).
Still a further object of the present invention is to provide an internally recirculating burner which is capable of burning a wide variety of fuels such as natural gas and a mixture of coal and residual oil in 50%--50% portions.
Another object of the present invention is to provide an improved recirculating burner design that operates over a wide range of fuel and air flow independently while maintaining flame at a single geometric setting of the nozzle and other components, thus providing ample turndown of the energy released by the burner and ample variations in the ratio of fuel to air.
Still another object of the present invention is to provide an improved internally recirculating burner design in association with the furnace system and wherein the burner is easily interchangeable with other burner designs.
A still further object of the invention is to combust a variety of fossil fuels within the furnace cavity so completely as to eliminate the "gray haze" normally generated by contemporary burners which interfers with the transmission of energy from the flame to the heat absorbing surfaces of the furnace by radiation.
A further object of the invention is to provide a burner which approaches a "perfect mixer." In such a burner, stoichiometric quantities of fuel and air are reacted to form the products of complete combustion without producing unburned hydrocarbons and releasing surplus oxygen (excess air) to carry out and waste the energy released by the combustion process. The subject of this invention approach is the perfect mixer concept, and it affords an opportunity to effectively use sorbents to absorb the sulfur which may be present in the fuel. Sulfur removal by the use of additives which would react in the flame zone to absorb the sulfur in the fuel has long been a goal of researchers. Laboratory demonstrations have shown the feasibility of employing sorbents directly in the fuel; however, in practice, such methods yield absorption rates of 75% to 80%, which is not adequate. Further, large quantities of the sorbent material have been required to accomplish these results. In most instances, 3-5 times the stiochiometric quantity of sorbent is required to obtain a level of 80% sulfur absorption.
Demonstrations have been performed with the invention wherein sorbents were employed at concentrations of 11/2 to 2 times the stoichiometric quantity. These demonstrations were performed with a 50%--50% mixture of coal and number 6 oil. The results of the test revealed that 96% of the sulfur in the fuel could be removed in the flame zone by the use of sorbents with the burner.